Various bone screws of this kind are known and can, for example, be used to stabilize bones, or fix them in place relative to each other, especially to stabilize and fix the spinal column, in that the respective bone screws are screwed into the bones to be fixed, such as vertebral bodies, and fastened by means of the fixing device, such as a rod-like longitudinal support for osteosynthesis, which is fixed in place on the associated fastening device.
A bone screw of this kind with fastening device is known from DE 199 44 120 A1. The fastening area of the bone screw for the fastening device is designed in the shape of a truncated cone formed on a partially spherical screw head, where the truncated cone, which expands preferably conically towards the screw shank, displays a threaded hole to accommodate a threaded screw, by means of which the fastening device, designed as a clamp, can be fixed in place on the screw head. This is intended to make it possible, even after positioning of the bone screw, for the location of the longitudinal support to be varied freely relative to its longitudinal axis.
However, it has proved to be disadvantageous with a bone screw of this kind that, when the bone screw is implanted, secure arrangement and simple handling of the clamps on the truncated cone during fixing in place and/or alignment of the spinal fixing element is not always possible in the desired way because of the projecting truncated cones. For example, with this arrangement, the clamp with the arch serving to accommodate the fixing rod is located very close to the spinous process of the vertebral body. Moreover, owing to the relatively low height of the truncated cone, there is a risk, in the event of pre-mounting of the clamps, of them slipping off the truncated cone. Furthermore, it is often necessary to deform the fixing rod to adjust it to the target position of the vertebral bodies. However, insertion of the connecting rod into the corresponding receptacles of the clamps is sometimes quite inconvenient with the known bone screw because of the given arrangement of the clamps. On the other hand, attachment of the connecting rod with pre-mounted clamps is difficult, especially in the event of long instrumentation extending over several vertebral bodies, because the clamps have to be levered over the truncated cones of the implanted bone screws.
In addition, DE 199 21 551 and EP 641 548 disclose bone screws with superficial toothing for fixing the fastening device in various angular positions. In this context, the toothing is usually provided over a large area on a convex section of the spherical head or on a plane of section of the sphere. However, the application of toothing of this kind is relatively complex and expensive.